Most types of optical scanners used in laser beam printers and the like have a configuration including a semiconductor laser as a light source, a first image forming optical system that linearly focuses a light beam from the light source on an optical deflector so as to compensate for the tilt of a deflection surface of the optical deflector, a polygon mirror as the optical deflector, a second image forming optical system that allows a uniform spot with a constant speed to be formed on a surface to be scanned, a scanning starting signal detector that detects a light beam scanned by the optical deflector, and a detecting optical system that condenses a light beam from the light source onto the scanning starting signal detector.
The second image forming optical system in a conventional optical scanner is configured of a so-called f-θ lens that is made up of a plurality of large-sized glass lenses, which has been a cause of difficulty in reducing size and high cost. With respect to this problem, in recent years, as disclosed in JP 8(1996)-94953 A and JP11(1999)-30710 A, optical scanners for achieving size and cost reduction that use a curved surface mirror as the second image forming optical system have been proposed.
However, with regard to each of the above optical scanners that have been proposed, though it is explained that ideally, a light beam from the curved surface mirror is guided directly to an image plane, since the light beam is reflected off the curved surface mirror at a small reflection angle, practically, in order for the light beam to be guided to a photosensitive drum constituting a surface to be scanned, the following configuration is required. That is, an optical path of an increased length is provided, and a polygon mirror, the curved surface mirror, and the photosensitive drum are disposed so that a distance between the polygon mirror and the curved surface mirror and a distance between the curved surface mirror and the photosensitive drum are increased. Because of this, particular schemes have been required to reduce the size of the scanners.